The goal of this proposal is to learn the rules by which neurons and their targets interact during development to achieve appropriate numerical balance and to establish the proper anatomical interconnections. The model system we have chosen to explore these issues is the olivocerebellar circuit of the laboratory mouse. During the coming funding period the emphasis will be on two basic issues: regulation of cell division and control of cell loss via cell death. There is ample evidence to suggest that the rate of division in cerebellar granule cell precursors is regulated by the proximity of their eventual target, the cerebellar Purkinje cell. This regulatory interaction is spatially heterogeneous, one consequence of which is the creation of the characteristic folding pattern of the cerebellar cortex. We will use three mutants two naturally occurring (staggerer and weaver) and one engineered (a gene knockout of the mouse engrailed-2 gene) - to question what features of the target is monitored to produce this broad scale pattern. Staggerer aggregation chimeras will enhance the study further. Labeling with BrdU followed by short survival times will reveal the most active regions of granule cell division and these will be compared with the location of the Purkinje cells in the different foliation patterns. Cell death will be explored in two main areas. In the first, we will continue our numerical matching studies in the olive->Purkinje and granule->Purkinje cell circuits. Data accumulated during the past funding period (from our lab and from others) has led us to hypothesize that different groups of olive cells have separate developmental relationships with unique groups of cerebellar Purkinje cells and, further, that each of these circuits has a different numerical matching function. This hypothesis will be corroborated by counts of olive and Purkinje neurons in three different inbred strains and their chimeras in cerebellar cortex we will use double-labeling (3H- thymidine/BrdU) of granule cell precursors in staggerer chimeras to determine if early born granule cells have a competitive advantage over later born afferents. Finally, an exciting new series of findings strongly implicates a loss of cell cycle control as a causative agent in the induction of target-related cell death. Both granule and inferior olive neurons in two neurological mutants (staggerer and lurcher) show evidence of induction of three different cell cycle markers before they die. The implications of this finding is that other neurodegenerative diseases including several human conditions might also involve this mechanism providing an analogy between induction of cell death and oncogenesis. This provocative finding will be explored first by enlarging the number of cell cycle markers to determine where in the cycle the cells reach before they die. T-antigen transgenic mice will also be produced under different neural specific promoters to assess the breadth of CNS cell types that will die if they are forced, as mature neurons, back into the cell cycle.